The presentation concerns the stabilization of organic polymers against free radicals or UV effects.
In particular the presentation concerns the use of cerium dioxide CeO2, especially in the form of discrete cerium dioxide particles, as an inorganic radical trap to stabilize organic polymers against degradation by free radicals, which can be generated especially by photochemical, thermal, physical and/or chemical induction or by exposure to UV (effect of ultraviolet radiation or ultraviolet light).
Moreover, the presentation concerns polymer compositions, especially in the form of polymer-based varnishes, paints, coating compounds or the like that contain cerium dioxide, especially in the form of discrete cerium dioxide particles, as inorganic radical trap.
In addition, the presentation concerns an agent for stabilizing organic polymers against photochemically, thermally, physically and/or chemically induced degradation by free radicals, especially against UV effects, and a method for stabilizing organic polymers, especially in the form of polymer-based formulations like varnishes, paints, coating compounds or the like, against such degradation by free radicals.
To stabilize organic polymers, for example in the form of polymer-based varnishes, paints, coating compounds or the like, against, for example photochemically induced, degradation by free radicals there are currently essentially four groups of substances or classes of substances in use:
For one there are organic UV-absorbers, for example those based on hydroxyphenylbenzotriazoles, hydroxybenzophenones, hydroxyphenyltriazines, oxyanilines, hydroxyphenylpyrimidines, salicylic acid derivates and cyanoacrylates.
Another group is organic radical traps, especially ones based on sterically hindered amines or on phenols.
In addition, peroxide-decomposing substances like thioethers, phosphites, etc., are used.
Finally, special inorganic fillers or pigments are used, for example ones based on zinc oxide, titanium dioxide, carbon blacks, iron oxides or the like, where especially filler particles with particle sizes in the range from 0.005 to 100 μm are used.
Said substances or classes of substances affect the process of degradation of inorganic polymers in view of the effect of ultraviolet radiation in different ways:
For instance, it is known that both the organic UV-absorbers of said kind and said inorganic fillers or pigments are capable of absorbing incident ultraviolet radiation via electron excitation and, in the ideal case, converting it to thermal energy.
However, limits are imposed on said reaction: the absorption of the UV radiation is not complete, so that unabsorbed UV radiation leads to radical-forming reactions within the polymer. However, the absorbed UV radiation also leads, via electron excitation processes, to radical-forming reactions within the polymer. Thus, all in all both unabsorbed and absorbed UV radiation leads to radical chain reactions in polymers, and bonds can be split, which can lead to degradation of the polymer matrix and, for example in the case of organically based lacquers, paints, coating compounds or the like, can also lead to degradation of the underlying substrate and moreover can cause color change processes, especially yellowing, of the polymers and/or the substrate.
In order to prevent said processes or at least to contain them to a large degree, organic radical traps of said kind are frequently combined with UV-absorbers today. The effects that arise from the radicals are largely neutralized by said substances, but the UV-absorbers and the organic radical traps gradually break down over time and thus lose effectiveness. Connected with that, a time-dependent effect in UV stabilization will always be observed where organic radical traps are combined with organic or even inorganic UV-absorbers.
For further details on the UV stabilization of paints, lacquers, coatings or the like, one can refer, for example, to A. Valet, Light Stabilizers for Paints, Curt R. Vincentz Publishers, Hannover, 1997 (ISBN 3-87870-443-7 Gb.).